This invention relates to centrifugal separators of the liquid powered kind for separating particulate contaminants from a liquid, such as a vehicle engine lubricant, within a containment rotor to which contaminated liquid is supplied at elevated pressure.
Liquid-powered centrifugal separator, are well known for separating fluids of different densities or for separating particulate matter from liquids and have long been used in lubrication systems for engines and analogous items of vehicles. The common principle of operation is that a housing contains a rotor which is supported therein to spin at high speed about a substantially vertical axis. The rotor comprises a container to which liquid is supplied at elevated pressure along the axis of rotation and is ejected from tangentially directed reaction jet nozzles into the housing from which it drains to the engine sump. Contaminated liquid lubricant is also supplied to the rotor to pass therethrough and, in doing so, denser contaminant materials are separated therefrom centrifugally and retained in the rotor. The drive liquid may comprise the contaminated liquid in a so-called self-powered centrifugal separator of the type described in, for example, GB-A-735658, GB-A-757538, GB-A-2160796, or GB-A-2383194, or it may be separate liquid as described in GB-A-2297499.
Notwithstanding the origin of the rotor drive liquid, the energy lost by the ejected liquid effects rotation of the rotor about the axis at a speed fast enough for the contaminated liquidcirculating in, and passing through, the rotor to deposit solid contaminants on surfaces that are spaced radially outwardly of the axis and face radially inwardly towards the axis. For efficient separation, and to ensure that separated contaminants do not interfere with the reaction jet nozzles, the rotor container may be provided with a radially inwardly extending partition wall that effectively divides the rotor into a separation chamber, in which the solids collect, and an outflow chamber, to which the cleaned liquid passes by way of a transfer aperture cited near the rotation axis. It is common in modem designs, such as, EP 0193000 and GB 2283694, for this partition wall to extend both radially and axially as what is sometimes referred to as a separation cone, which better holds solids and liquid-containing sludge within the separation chamber if the rotation axis is tilted from the vertical.
There are several criteria associated with successful operation. In particular, liquid supplied to the rotor to drive it has to be available at a significant pressure if the energy lost by its passage through the reaction jet nozzles is to be sufficient to rotate the rotor fast enough to effect centrifugal separation of said contaminated particles. Also, the rotor has to be supported within the housing enclosure by bearing means permitting its rotation and therefore the efficiency of rotation, and thus of separation, depends upon inefficiencies and losses within the bearing means. Furthermore, there is a tendency in certain circumstances for the rotating rotor to give rise to vibration, particularly as noise, that is transmitted to the housing by way of the bearing means, which vibration not only represents an energy loss detracting from rotation efficiency but also causes disturbance to the comfort of users of the vehicle. The problems of vibration is most acute when the spinning rotor winds down after the supply liquid pressure ceases, as discussed further below, and has been discussed in GB-A-2308557, the contents of which are incorporated by reference.
Conventionally the rotor is mounted by way of axle means in the form of a static spindle that is fixed with respect to the housing and relative to which the rotor rotates, or in the form of a spindle that is fixed to, and rotatable with, the rotor relative to the housing, and for convenience and clarity within this specification, such a rotatable spindle is referred to as a xe2x80x98shaftxe2x80x99 that extends from the ends of the rotor.
It will be appreciated that the above described static spindle and rotatable shaft may each be formed extending completely through the rotor or as short stubs extending from the housing and/or rotor as appropriate and the terms xe2x80x98axle meansxe2x80x99, xe2x80x98spindlexe2x80x99and xe2x80x98shaftxe2x80x99is intended to incorporate such variants.
Notwithstanding the form taken by the axle means, the centrifuge rotor is usually mounted for rotation by bearings comprising plain, parallel bushes carried at each end of the rotor and surrounding the vertically extending axle means to form journal bearings. The bushes are a clearance fit on the axle means to permit unimpeded rotation and the gap between each bush and axle means is exposed to the lubricant supplied to the rotor such that some lubricant escapes along the gap and with the rotation creates a hydrodynamic film that provides easy rotation and some radial stiffness.
Mounting of the rotor by way of plain journal bearing bushes for rotation about a vertical axis provides no axial support per se for the axially directed load exerted on the rotor during operation. Whereas a limited degree of axial displacement is appropriate to facilitate free rotation, there is provided thrust bearing means to support an axially directed load on the rotor during operation, and possibly also displacement stop means operable to limit axial displacement in the axial direction opposite to that accommodated by the thrust bearing means, which displacement stop means may, of course, take the form of a supplementary thrust bearing to support the rotor in specific circumstances with minimal energy loss.
Conventionally such thrust bearing means and supplementary thrust bearing means is provided by a radially extending flanged end to one of the journal bushes and a facing surface fixed with respect to the housing, whereby the pressurised lubricant that provides lubrication and radial stiffness to the journal bearings also provides lubrication to the thrust bearing means.
In its simplest form the thrust bearing means is disposed between the rotor to support the axial load due to the combined weight of the rotor structure and the liquid lubricant filling it, including possibly an effective increase in such loading that comes from externally applied acceleration or shock forces. Such thrust bearing means is for convenience herein referred to as xe2x80x98weightxe2x80x99 thrust bearing means.
It will be appreciated that the axial loading placed on such weight thrust bearing means by the filled rotor may be several kilograms and is much greater than loading on the journal bearings, giving potential for excessive energy loss in, and even wear of, the thrust bearing means. The problems of axial loading due to rotor weight has been addressed by effecting so-called pressure-induced lift whereby the journal bushes exposed to drive liquid supply pressure are of different wall thicknesses so that the liquid pressure acts on the ends of the bushes and exerts a lifting force on the rotor to counter its weight. In centrifrugal separators that are of heavyweight construction and wherein the rotors are heavy and/or contain a large volume of liquid lubricant, such pressure induced lift as is feasible may only serve to reduce the effective axial load on the weight thrust bearing. In separators employing smaller, lighter rotors the degree of lift achievable may be such that it can completely overcome the weight of the filled rotor and the main thrust bearing means may be sited above the rotor whereby, in normal operation, an axial load on the rotor that is of lesser magnitude than the weight of the filled rotor bears upwardly on the thrust bearing means. Such style of thrust bearing means may be referred to for convenience as xe2x80x98pressure liftxe2x80x99 thrust bearing means. Clearly in the absence of adequate drive liquid supply pressure the rotor will be subjected to an overall downwardly directed loading and will tend to downward displacement. Accordingly, the provision of displacement stop means (conveniently below the rotor) is appropriate, possibly as supplementary thrust bearing means.
In addition to the load carrying capacity of the thrust bearing means being dependant upon drive liquid supply pressure, the radial stiffness of the journal bearings is also sensitive to such pressure and various external or out-of-balance forces acting on the rotor can result is disruption of the hydrodynamic film and contact between the relatively moving parts, which is a source of energy loss, wear and vibration/noise. As mentioned above, this is particularly troublesome when the lubricant supply ceases and the rotor winds down to rest. Also the absence of a replenished lubricant film between component parts of the thrust or supplementary thrust bearing permits physical abutment of the relatively rotating parts that can further be a source of vibration/noise during a prolonged wind-down interval.
It is an object of the present invention to provide a centrifugal separator which is capable of smaller bearing losses than hitherto in normal operation, and furthermore is capable of providing reduced levels of noise and vibration in normal operation and/or on wind down.
According to the present inventions a centrifugal separator comprises a housing enclosure, an axis extending through the housing enclosure in an operationally substantially vertical orientation, a rotor arranged to receive a liquid at elevated pressure and rotatable about the axis in reaction to ejection of the liquid therefrom substantially tangentially, journal bearing means to facilitate said rotation of the rotor about the axis and thrust bearing means to support the rotor in respect of displacement along the direction of the axis, the thrust bearing means comprising at least one magnetic repulsion thrust bearing operable to support by magnetic repulsion between two magnetic elements thereof at least part of the load created by the operatively rotating rotor in a direction along the axis.
The magnetic repulsion thrust bearing may comprise a xe2x80x98weightxe2x80x99 thrust bearing to support the weight of the liquid-filled rotor and/or a xe2x80x98liftxe2x80x99 bearing to limit upward displacement of the liquid filled rotor due to drive liquid supply pressure in normal operation. Furthermore, it may comprise axial displacement stop means to limit displacement of an empty or emptying rotor acted on by a magnetic repulsion thrust bearing.
Embodiments of the invention will now be described by way of example with reference to the accompanying drawings.